In data links, the transmission path used for transmitting signals is known to cause interference on telecommunication. This occurs regardless of the physical form of the transmission path, i.e. whether the path is a radio link, an optical fibre or a copper cable.
In order to diminish the effects of interference caused by the transmission path, a digital signal is encoded so that the connection could be made more reliable. In such a case, the errors caused by the interference in the signal to be transmitted can be detected and also corrected without retransmission, depending on the encoding method used.
Conventional coding methods used in digital telecommunication include for example block coding and convolutional coding. In block coding, the bits to be encoded are grouped into blocks and parity bits are added at the end of the blocks so that the correctness of the bits in the preceding block can be checked by means of the parity bits. In convolutional coding, the parity bits are placed among the data bits so that the encoding is continuous. The data bits are not grouped into blocks nor are the parity bits connected to the immediately preceding data bits, but they are distributed over the area of a bit group of a certain length, this bit number being called the constraint length of the convolutional code. Convolutional encoders and decoders are implemented in manners known in the field. An encoder can be realized for example with shift registers.
"Very Low Rate Convolutional Codes for Maximum Theoretical Performance of Spread-Spectrum Multiple-Access Channels", Viterbi, IEEE Journal on Selected Areas in Communications, Vol. 8, No. 4, May 1990, pp. 641-649, discloses a method which combines convolutional coding and multiple-level orthogonal modulation. In this method, encoding takes place in principle as shown in FIG. 1. The encoder shown in the figure consists of a k-bit convolutional encoder that is realized with a shift register 100 and that outputs m code bits 106 which control an orthogonal modulator 102 the output of which produces one of M=2.sup.m possible orthogonal symbols, which is shown with M parallel bits. This M-level orthogonal signal group may be for example an M-level Walsh signal group. FIG. 2 shows a decoder realizing the method according to the aforementioned reference and comprising an M-level demodulator 200, for example a group of correlators or a Walsh-Hadamard conversion circuit, the output of which consists of M correlation values 204 that are supplied to a Viterbi decoder 202.
U.S. Pat. No. 5,193,094 discloses a method similar to the previous method, wherein the first and the last one of the bits in the output of a shift register are taken and connected to an XOR gate the output of which is supplied with the output of an orthogonal modulator to another XOR gate the output of which is the symbol to be transmitted.
Near Shannon limit error-correcting coding and decoding: Turbo-codes by Berrou, C., Glavieux, A., Thitimajshima, P., IEEE International Conf. on Communications, ICC'93, Geneva, Switzerland 23-26 May, 1993, Vol. 2, pp. 1064-1070, which is incorporated herein by reference, describes the principle of parallel concatenated convolutional coding. These codes are often called turbo-codes. The capacity of turbo-codes in an AWGN channel is excellent. The encoding takes place with two or more parallel encoders, and the signals input to the encoders are interleaved in order to produce an independent data flow.
The drawback of the known methods is that they require the received signal to have a higher signal-to-noise ratio than the novel arrangement according to the invention in order to achieve the desired quality of transmission.